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Abstract:

A method for determining the physical location of a fiber optic channel
in a fiber optic cable comprises the steps of a) providing at least one
location key having a known physical location, b) establishing the
location of the location key with respect to the fiber optic channel, and
c) using the location information established in step b) to determine the
physical location of the channel. The location key may comprises an
acoustic source, a section of fiber optic cable that is acoustically
masked, or at least one magnetic field source and step b) comprises using
a Lorentz force to establish the location of the magnetic field source
with respect to the fiber optic channel.

Claims:

1. A method for determining the physical location of a fiber optic
channel in a fiber optic cable comprises the steps of: a) providing at
least one location key having a known physical location; b) establishing
the location of the location key with respect to the fiber optic channel;
and c) using the location information established in step b) to determine
the physical location of the channel.

2. The method according to claim 1 wherein the location key comprises an
acoustic source and step b) comprises using time as a basis for to
correlating a signal transmitted by the acoustic source to a signal
received in the fiber optic channel.

3. The method according to claim 2 wherein the location key comprises an
acoustic source that moves relative to the fiber optic cable.

4. The method according to claim 2 wherein the fiber optic cable is in a
well and the location key comprises an acoustic source that is lowered
into the well.

5. The method according to claim 2 wherein the location key comprises at
least one acoustic source that does not move relative to the fiber optic
cable.

6. The method according to claim 1 wherein the location key comprises a
section of fiber optic cable that is acoustically masked and step b)
comprises using ambient noise to establish the location of the
acoustically masked section with respect to the fiber optic channel.

7. The method according to claim 1 wherein the location key comprises at
least one magnetic field source and step b) comprises using a Lorentz
force to establish the location of the magnetic field source with respect
to the fiber optic channel.

8. The method according to claim 7 wherein an electrical conductor is
deployed near the fiber optic cable and step b) comprises passing a
current through the electrical conductor so as to cause it to deform as a
result of the magnetic field generated by the magnetic field source.

9. The method according to claim 8, further comprising using an optical
signal and OTDR in the fiber optic channel to detect the deformation.

10. The method according to claim 7 wherein the magnetic field source is
fixed with respect to the fiber optic cable.

11. The method according the claim 1 wherein step a) has the location key
comprised of the movement of a tool hitting or rubbing against the inside
of the well-bore and step b) comprises of the time-depth pairs given by
the contractor of the location of the tool with respect to time

[0002] The present disclosure relates generally to a system and a method
for improving the accuracy of location measurements made using fiber
optic cable and backscattered light.

BACKGROUND OF THE INVENTION

[0003] The use of backscattered light in fiber optic cables has found
increasing acceptance in a variety of applications. Because light can be
backscattered from any location along the length of a fiber, information
can be obtained over significant distances and such systems are often
referred to as "distributed" sensors. Because distortion or deformation
of the fiber can be sensed, distributed sensors comprised of fiber optic
cable can be used to sense temperature, pressure, strain, acoustic
events, and the like. Distributed systems have been used advantageously
in oilfield applications, in traffic monitoring, and in military/security
applications, among others.

[0004] In a typical fiber optic-based distributed sensing system, one or
more fiber optic cables designed to collect distributed strain
measurements are deployed in a desired location and coupled to the
sensing subject by suitable means. One or more light boxes containing
laser light sources and signal-receiving means are optically coupled to
the fiber. In some embodiments, the light source may be a long coherence
length phase-stable laser and is used to transmit direct sequence spread
spectrum encoded light down the fiber. The cable may be double-ended,
i.e. may be bent in the middle so that both ends of the cable are at the
source, or it may be single-ended, with one end at the source and the
other end at a point that is remote from the source. The length of the
cable can range from a few meters to several kilometers, or even hundreds
of kilometers. In any case, measurements can be based solely on
backscattered light, if there is a light-receiving means only at the
source end of the cable, or a light receiving means can be provided at
the second end of the cable, so that the intensity of light at the second
end of the fiber optic cable can also be measured.

[0005] When it is desired to make measurements, the light source transmits
at least one light pulse into the end of the fiber optic cable and a
backscattered signal is received at the signal-receiving means. Localized
strain or other disruptions cause small changes to the fiber, which in
turn produce changes in the backscattered light signal. The returning
light signal thus contains both information about the deformation of the
fiber and location information indicating where along the fiber it
occurred. Known optical time-domain reflectometry (OTDR) methods can be
used to infer information about the sensing subject based on the
backscattered signal from one or more segments of the fiber adjacent to
the subject. Typically, the location of the backscattering reflection at
a point along the fiber can be determined using spread spectrum encoding,
which uniquely encodes the time of flight along the length of the fiber,
dividing the fiber into discrete channels along its length.

[0006] In some applications, including downhole applications, the physical
channel depths cannot practically be measured directly, but they can be
roughly inferred on the basis of timing and fiber refraction index, i.e.
the "optical depth." These rough calculations are not sufficiently
precise for some purposes, however, because they incorporate
uncertainties that, while small on a percent scale, build to a
significant magnitude over the length of the fiber. For downhole seismic
applications, repeatable physical depth positioning of the channels
within an accuracy of 1 meter or better is desired.

[0007] Currently, however, there is no practical way to determine the
actual physical location of a given backscattered signal. Hence, there
remains a need for a system that would allow the physical location of a
given backscattered signal to be determined with the desired accuracy
and, if possible without requiring re-entry of the well at a later date
to measure channel drift.

SUMMARY OF THE INVENTION

[0008] The present invention provides a systematic and reliable location
measurement that can be repeated over time, optionally using only
measurements made only at the lightbox.

[0009] The invention includes a method for determining the physical
location of a fiber optic channel in a fiber optic cable comprising the
steps of a) providing at least one location key having a known physical
location, b) establishing the location of the location key with respect
to the fiber optic channel, and c) using the location information
established in step b) to determine the physical location of the channel.
The location key may comprise an acoustic source, a section of fiber
optic cable that is acoustically masked, or at least one magnetic field
source.

[0010] In instances where the location key comprises a magnetic field
source, step b) may comprise using a Lorentz force or other
electromagnetic effect, such as magnetostrictive force, to establish the
location of the magnetic field source with respect to the fiber optic
channel. The Lorentz force can be applied by including an electrical
conductor deployed near the fiber optic cable and passing a current
through the electrical conductor so as to cause it to deform as a result
of the magnetic field generated by the magnetic field source. In these
instances, an optical signal can be used in conjunction with OTDR in the
fiber optic channel to detect the deformation and thereby determine
location of the channel. The magnetic field source may or may not be
fixed with respect to the fiber optic cable.

[0011] In instances where the location key comprises an acoustic source,
step b) may comprise using "time-of-day" measurement to correlate a
signal transmitted by the acoustic source to a signal received in the
fiber optic channel. With this method, when the location key is at a
known depth, the acoustic source excites a particular channel and the two
can be correlated via independent measurements of the time-of-day noted
for the depth measurement and the time-of-day noted for the channel
measurement. The location key may or may not move relative to the fiber
optic cable. In some cases, the fiber optic cable may be disposed in a
well and the location key may comprise an acoustic source that is lowered
into the well, recording either when moving into the well, moving out of
the well or both.

[0012] In instances where the location key comprises a section of fiber
optic cable that is acoustically masked, step b) may comprise using
ambient noise to establish the location of the acoustically masked
section with respect to the fiber optic channel.

[0013] In some embodiments, the invention includes masking of intermittent
sections of the cable so that the masked sections can be detected using
only ambient noise. If the physical positions of the masked segments are
known accurately, the calibration between OTDR two-way-light-travel time
and cable location can be continuously calibrated simply by observing the
masked segments.

[0014] Embodiments of the invention may also include interpolation of
depth between total depth and surface and reliance on noise made when
downhole tools hit casing collar locators as the well is logged.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a more complete understanding of the preferred embodiments,
reference is made to the accompanying drawing, which is a schematic
illustration of a system in accordance with a first embodiment of the
invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0016] The present disclosure relates generally to a system and a method
that allows the physical location of backscattered signals in a fiber
optic cable to be determined with precision.

[0017] Referring initially to the FIGURE, a well 10 contains a fiber optic
cable 12 that follows the well. Cable 12 is optically coupled at one end
to a light box 14, such as are known in the art. Cable 12 may be
double-ended, i.e. may be bent in the middle so that both ends of the
cable are at the light source, or it may be single-ended, with one end at
the source and the other end at a point that is remote from the light
source. In the embodiment illustrated in the FIGURE, well 10 contains a
tubular 11, such as a casing or liner. Cable 12 is run into the well in
conjunction with tubular 11 and cement is pumped into the annulus between
the tubular and the wellbore, thereby mechanically coupling cable 12 to
the formation.

[0018] It will be understood by those skilled in the art that there are
many suitable techniques by which cable 12 can be coupled to the
formation or other subject environment. In the case of well 10, cable 12
can be clamped to tubular 11 or adhered to its inner or outer surface,
either in a groove, or not. Cable 12 can be emplaced with the cement or
lowered via a wireline. In the latter instance, cable 12 may or may not
be subsequently mechanically affixed to the tubular.

[0019] Even though, as described above, the cable can be optically
interrogated and the resulting optical signal used to divide the fiber
into sections or "channels" whose nominal distances from the light box
are known, it is not possible to know precisely where each channel is
located in relation to the physical environment.

[0020] According to the present invention, at least one location key
having a known physical location is provided and its location with
respect to at least one fiber optic channel is established, thereby
allowing the physical location of the channel to be determined.

[0021] One way to determine the physical location of each channel is to
lower into well 10 an acoustic signaling instrument (not shown) that
transmits an intermittent or continuous acoustic signal and serves a
location key. If the position of the instrument is tracked as a function
of time, the distributed signals from the fiber that are recorded over
the same time period can be correlated to the signal position
information. Examples of suitable noise-transmitting means include but
are not limited to: gauge collars, which generate noise by rubbing
against the borehole, wireline sonic tools, cement bond logging tools,
and the like. While this technique is described and has particular
utility in a borehole, it will be understood that a similar correlation
can be performed on any combination of time-stamped physical and optical
data, including surface and marine applications.

[0022] In a variation of the above technique, an acoustic signal can be
generated by movement of the tool, rather than by the tool itself. Thus,
for example, a wireline tool that is being lowered in to a borehole will
tend to generate an identifiable acoustic signal while rubbing against
the side of the hole or when it hits each pipe joint. The tool, whose
source location is known, can serve as a location key and the signals it
generates can be used to determine the location of each fiber optic
channel.

[0023] The foregoing method provides information about the location of
each channel at a given point in time. So long as the channels do not
move, signals received from each channel can be relied on to provide
location information. Over extended time periods, however, it may be
expected that changes in the optical properties of the fiber may result
in movement or change in the actual location of one or more of the
channels as-detected. The length of such time periods can depend on the
nature of the fiber, the materials from which it is constructed, and the
environment in which it is deployed, and can be on the order of 1, 10,
50, or more years. Thus, in instances where it is anticipated that the
fiber will be used for a protracted period, one or more sections 16 of
cable 12 may be acoustically masked or otherwise decoupled from their
environment, as illustrated schematically in the FIGURE. The length of
each masked section 16 is preferably relatively short compared to the
total length of cable 12 but preferably no smaller than the length of an
individual fiber optic channel. By way of example only, masked section(s)
16 may be 1, 10, or 20 meters long. The length of masked segments 16 may
be constrained by the resolution of the sensing apparatus.

[0024] Because they are acoustically decoupled from their environment,
sections 16 will be visible on a DAS system in the presence of ambient
noise, even if no other noise-generating operations are occurring. Thus,
if the actual positions (depths) of the masked segments are known
accurately, each masked section 16 can serve as a location key and the
calibration between OTDR time and cable depth can be calibrated by
observing the segments in a conventional DAS measurement. Still further,
if the actual positions of masked segments do not change as a function of
time, the calibration between OTDR time and cable depth can be
continuously monitored. If the actual positions of masked segments do
change over time, the relationship between OTDR time and cable depth can
be re-calibrated.

[0025] As with the fiber optic channels, the physical positions of the
masked segments can be initially determined and/or subsequently
re-determined in several ways. By way of example only, each masked
section may contain a weak radioactive source that can be detected in a
gamma ray log after fiber installation. That gamma ray (GR) log could
then be correlated to formation properties and/or a casing collar locator
(CCL) log. Alternatively, the acoustic logging-like locating techniques
described above can be used to initially establish the location of the
deployed segments. In order to properly observe the masks using DAS OTDR,
with maximum resolution, it may be necessary to position the pulse(s)
directly over the mask in an iterative procedure.

[0026] Cable masking can be provided by including a variable coating on
the fiber or by including variations in the cable itself. By way of
example only, masked sections 16 can be provided in a gel-filled cable by
including sections that have no gel fill. These are preferably created
during the cable manufacturing process. In other embodiments the masking
can be applied as the cable is deployed, such as by applying a layer of
foamed or otherwise acoustically-isolating material. Cable masking other
than acoustic is also contemplated, including for example, the inclusion
in the cable of a material having varying radial thermal conductivity
along the length of the cable.

[0027] In still other embodiments, (not shown) one or more localized
magnetic field sources are placed in proximity to the fiber and an
electrical conductor such as a conducting wire is also placed in
proximity to the fiber. In one embodiment, a plurality of localized
magnetic field sources is deployed in a spaced-apart manner along the
length of a fiber. When a current is passed through the wire, the
magnetic field generated by each magnetic field source will cause a
force, called the Lorentz force, to be applied locally to the wire. The
direction of the force is orthogonal to both the electric current and
magnetic field. In the present application, the magnetic field is
preferably anisotropic and arranged to be orthogonal to the wire. Thus,
the Lorentz force will be orthogonal to the wire, with the result that
the wire will be locally curved when current is flowing through the wire.
A Lorentz force can be generated using either DC or AC currents, with the
effect that vibrations or variable and tunable frequency can be
generated.

[0028] Because the fiber optic sensors are extremely sensitive, the small
deformation of the wire resulting from the application of the Loretz
force can be detected using OTDR techniques. Thus, if the physical
location of the of the deformation (magnetic field source) is known, each
magnetic field source can be used as a location key, i.e. used to
calibrate the physical locations of the fiber optic channels. As
discussed above, a wireline or similar tool can be used to sense and
locate each magnetic field source. In a variation on this embodiment, the
magnetic field source(s) can be provided separately from the fiber. In
this variation, a conducting wire is preferably included with or near the
fiber and one or more magnets is moved along the fiber. The localized
magnetic field will cause a localized deflection of the conducting wire,
which can in turn be detected using OTDR techniques.

[0029] When the exciting current is switched off, the fiber and DAS system
performance will be unaffected. When the exciting current is switched on,
the fibre and DAS system will only be subjected to vibrations where the
magnetic sources are located and not at other locations.

[0030] By way of example only, the magnetic field sources may be neodymium
magnets and the fiber optic cable may be encapsulated in an optional
metal tube that is transparent to magnetic fields. Further by way of
example, magnetic field sources can be built into traditional tubing
clamps that are used to retain cables during and the fibre cables can be
manufactures to include a conductive wire.

[0031] Like the other techniques described above, the Loretz-force
technique can be used to mark locations on the fibre for depth
calibration purposes that will not change with time.

[0032] As is known in the art, if the location of each sensor is known,
the fiber optic channels can be interrogated in the time scale of
fractions of a millisecond, providing a virtually instantaneous
measurement at all depths of interest. The information gained in this
manner can be used to diagnose and correct a geomechanical model or can
be used to directly intervene in the treatment with or without
integration with other measurements.

[0033] The present methods have no inherent lower limit to the frequency
of investigation and are therefore limited only by the stability of the
hardware over long time scales. There are various methods of backscatter
measurement, including the use of Rayleigh and Brillouin backscattering,
and one method may be preferred over others for this implementation of
the present invention, especially at low frequency.

[0034] Illustrative embodiments of the present claimed subject matter have
been described in detail. In the interest of clarity, not all features of
an actual implementation are described in this specification. It will be
understood that in the development of any such actual embodiment,
numerous implementation-specific decisions must be made to achieve the
developers' specific goals, such as compliance with system-related and
business-related constraints, which will vary from one implementation to
another. Moreover, it will be appreciated that such a development effort
might be complex and time-consuming, but would nevertheless be a routine
undertaking for those of ordinary skill in the art having the benefit of
the present disclosure. In the claims, unless explicitly stated, the
sequential recitation of steps is not intended to require that the steps
be performed sequentially.

[0035] In still other variations on the foregoing embodiments, optical
channels can be located by tracking a tool that generates noise either by
rubbing the casing, or sending deliberate pulses or hitting the casing
collars. These `calibration` points provide a physical depths because
either the time-stamp is know or the actual depth is measured. Another
method for locating channels is by changing something in the cable
construction (the mask or the magnate) and transmitting through the cable
or casing a signal that does not require the intervention. Another method
for locating channels is by creating a pressure pulse in the wellbore
that creates a `wave` down the wellbore with fluid velocity, rock
velocity and steel velocity; the wave would show reflected signals at
interfaces whose positions were known (e.g. casing shoes, casing collars,
cement tops, formation tops etc.).

[0036] The particular embodiments disclosed above are illustrative only,
as the present claimed subject matter may be modified and practiced in
different but equivalent manners apparent to those skilled in the art
having the benefit of the teachings herein. Furthermore, no limitations
are intended to the details of construction or design herein shown, other
than as described in the claims below. It is therefore evident that the
particular illustrative embodiments disclosed above may be altered or
modified and all such variations are considered within the scope of the
claims. By way of example only, one of skill in the art will recognize
that the number and location of the location key(s), the manner for
determining location key position, the number and configuration of cables
and sensors, the sampling rate and frequencies of light used, and the
nature of the cable, coupling devices, light sources and photodetectors
can all be modified. Accordingly, the protection sought herein is as set
forth in the claims below.

Patent applications by Daniel Joinson, Rijswijk NL

Patent applications by Menno Mathieu Molenaar, Calgary CA

Patent applications by SHELL OIL COMPANY

Patent applications in class Including physical deformation or movement of waveguide

Patent applications in all subclasses Including physical deformation or movement of waveguide